This project is concerned with the development and use of analytical procedures for identification and characterization of gonadotropin receptor sites in the testis and ovary. Methods of gonadotropin labeling, characterization of the labeled tracer, and optimization of receptor assay methods are evaluated and improved. Computerized methods for analysis of binding data, from kinetic and equilibrium experiments, plus corrections for the tracer characteristics, are applied to studies on the properties of gonadotropin receptors. Native and modified gonadotropins are evaluated for receptor binding activity, and receptors are analyzed in animal and human tissues.